Field of the Invention
The invention lies in the field of chemical engineering and environmental protection. Specifically, the present invention relates to a method of introducing at least one second reaction medium into at least one first reaction medium flowing in a channel and for intensive mixing of the reaction media with one another.
The present invention relates in particular to a process for the removal of pollutants, especially nitrogen oxides, contained in exhaust gases produced in the firing of fossil fuels or in the incineration of waste. The pollutants are removed by reduction with the use of a catalyst, a reducing agent being introduced into the exhaust gases and being mixed with the exhaust gases. The exhaust gases together with the reducing agent are conducted over the catalyst, as a result of which the pollutants are converted into environmentally neutral gases.
The present invention also relates to a device for carrying out these aforementioned processes.
In chemical process engineering, there is often a need to introduce one or more second reaction media into at least one flowing first reaction medium and to mix the two reaction media intensively with one another within a short distance of travel. This need arises, for example, in the firing of fossil fuels such as coal, oil and gas, and in the incineration of waste, pollutants. The primary noxious gas thereby are nitrogen oxides NOx, which must be removed from the exhaust gases.
It is known, to that end, to introduce a reducing agent into the exhaust gases and then feed the exhaust gases intensively mixed with the reducing agent over a catalytic converter with catalyst material. As a result, the pollutants are converted into environmentally neutral gases. These reactions take place at temperatures of about 150xc2x0 C. to about 450xc2x0 C.
By adding NH3 as a reducing agent for example, it is possible to convert NO and NO2 into N2 and H2O.
For this principle in the elimination of NOx on a catalyst, the following basic reaction kinetics apply:
adsorption of an ammonia gas molecule on a vanadic acid molecule;
formation of an ammonium meta-vanadate complex;
reaction of the ammonium meta-vanadate complex with a nitrogen oxide molecule;
desorption of the reaction products;
oxidation of the catalyst into its active initial state with the aid of the oxygen in the exhaust gas.
It is understood in this context that the effectiveness of this process is strongly dependent, on the one hand, on the degree of mixing of the ammonia and nitrogen oxide molecules and, on the other hand, on their uniform distribution throughout the flow of the exhaust gases.
In order to respond to the requirements of mixing with and distributing a reducing agent in the exhaust gases, it is known to design the exhaust gas channel with lances which project into it and are equipped with a multiplicity of nozzles, the individual nozzle lances being supplied individually. However, this further leads to the requirement of providing a sufficiently large spacing between the positions where the reducing agent enters the exhaust gases and the catalyst, in order thereby to achieve the required high degree of mixing of the reaction media with one another. The greater the space between the inlet of the reducing agent and the catalyst, the greater the degree of mixing.
In order to improve the degree of mixing further, it has also been known in the art to provide static mixing devices, such as for example baffle detours and chicanes for deflecting the exhaust gases mixed with the reducing agent, or to arrange heating devices in the exhaust gas channel. These known measures are, however, a disadvantage because on the one hand they entail extra outlay and on the other hand they require extra space.
It is accordingly an object of the invention to provide a process for introducing at least one second reaction medium into at least one first reaction medium flowing in a channel, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for improved mixing within a limited space.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of introducing a second reaction medium into and mixing the second reaction medium with a first reaction medium, which comprises:
conducting the first reaction medium in a flow channel;
introducing the second reaction medium at a plurality of locations strategically distributed about a cross section of the flow channel into the first reaction medium; and
inducing turbulence in the first reaction medium at the locations at which the second reaction medium is introduced and thereby intensely intermixing the first and second reaction media directly at the locations at which the second reaction medium is introduced.
In accordance with an added feature of the invention, the second reaction medium is introduced at a multiplicity of locations strategically distributed about the cross section of the flow channel.
In accordance with an additional feature of the invention, the first reaction medium is a pollutant-laden (e.g. nitrogen oxides NOx) exhaust gas produced in a combustion of fossil fuels or an incineration of waste and the second reaction medium is a reducing agent, and the method further comprises, subsequently to the intensely mixing step, conducting the exhaust gas containing the reducing agent through a catalyst and converting the pollutants in the exhaust gas into environmentally neutral gases.
In accordance with another feature of the invention, the inducing step comprises causing reduced pressure and inducing turbulence flows in the first reaction medium at the locations at which the second reaction medium is introduced.
In other words, at least one reaction medium is introduced at a plurality or a multiplicity of positions distributed over a cross section of the flow channel into the at least one first reaction medium. Turbulence flows (such as eddie currents) are induced in the at least one first reaction medium at the positions where the at least one second reaction medium is introduced. The intense mixing of the reaction media with one another therefore takes place directly in the regions where the at least one second reaction medium is introduced.
It is a specific object of the present invention to provide a process for purifying the exhaust gases produced in the burning of materials, as a result of which the outlay on extra devices is kept as low as possible and which guarantees such intensive mixing of the reducing agents with the exhaust gases that the exhaust gases can be fed over a catalyst directly thereafter, without thereby compromising the efficiency of the reduction.
These objects are satisfied in that turbulence is induced in is the exhaust gases containing pollutants in the regions where the reducing agent is introduced. As a result, the required intensive mixing of the exhaust gases with the reducing agent is brought about directly in the regions where the reducing agent is introduced into the exhaust gases.
The turbulence mixing is preferably induced by causing reduced pressure regions and attendant turbulence at the defined locations.
With the above and other objects in view there is also provided, in accordance with the invention, a device for carrying out the above-described method. The novel device comprises:
a flow channel for conducting the first reaction medium containing pollutants in a given flow direction;
an introducing and mixing device (i.e. a reducing agent metering system) formed with a plurality of nozzle apertures for introducing the second reaction medium strategically disposed in the flow channel and a plurality of deflectors projecting transversely to the flow direction of the first reaction medium and operatively associated with the nozzle apertures; and
a catalytic converter disposed downstream of the device in the flow direction of the reaction media for catalytically inducing a reaction between the reaction media.
In a preferred embodiment, the flow channel is an exhaust duct conducting exhaust gases originating in a combustion of fossil fuels or an incineration of waste. In that case, the second reaction medium is a reducing agent for reducing the pollutants (e.g. nitrogen oxides) in the exhaust gas.
In accordance with a further feature of the invention, the mixing device comprises a plurality of tubular pieces each having a free end formed with the nozzle apertures, and the deflectors are fastened to the tubular pieces.
In accordance with again an added feature of the invention, the mixing device comprises a plurality of tubes disposed in the flow channel transversely to the given flow direction for feeding the second reaction medium into the channel, and a plurality of tubular pieces fluidically communicating with and projecting from the tubes, the tubular pieces having free ends each formed with at least one of the nozzle apertures, and wherein the deflectors project transversely from the tubular pieces at a location upstream and in vicinity of the at least one nozzle aperture in the given flow direction.
In accordance with again an additional feature of the invention, the tubular pieces enclose an angle of between about 30xc2x0 and 60xc2x0, and preferably of about 45xc2x0, with the given flow direction and the deflectors project perpendicularly from the tubular pieces.
In accordance with again a further feature of the invention, the deflectors extend over at least 10% over the area of the cross section of the flow channel.
In accordance with a concomitant feature of the invention, the mixing device includes a plurality of lines disposed in form of a grille and projecting into the flow channel for feeding the second reaction medium, and the tubular pieces formed with the nozzle apertures and the deflectors project from the plurality of lines.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for introducing at least one second reaction medium into at least one first reaction medium flowing in a channel and for intensive mixing of the reaction media with one another, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.